hmm, that is useful (and logical for cvs and svn) but i did not yet notice such a feature with git. at least by default it has no concept of keywords but even if it could ignore keyword values in a diff, it could only do so if keyword expansion is activated for which (i believe) it would be necessary to remove the keyword values on checkin. (which is the issue we are discussing)
From gitattributes(5):
ident When the attribute ident is set for a path, git replaces $Id$ in the blob object with $Id:, followed by the 40-character hexadecimal blob object name, followed by a dollar sign $ upon checkout. Any byte sequence that begins with $Id: and ends with $ in the worktree file is replaced with $Id$ upon check-in.
My converter automatically sets this attribute for cvs files that had keywordexpansion enabled.